A resistance random access memory (ReRAM) is drawing attention and being developed as a memory to replace DRAM (Dynamic Random Access Memory) and flash memory. The cell structure of ReRAM is a simple capacitor structure in which a variable resistance film is interposed between two metal electrodes; and information can be stored by changing the resistance of the variable resistance film with an on/off ratio of about ten to ten thousand by applying a pulse voltage to cause a current to flow.
Methods for operating ReRAM include the two types of a unipolar operation that changes the resistance by applying voltages of the same polarity and a bipolar operation that changes the resistance by applying voltages of different polarities. ReRAMs having bipolar operations include a metal filament-formed ReRAM that changes the resistance by applying a voltage to ionize the metal and cause metal ions to diffuse and precipitate inside the insulative variable resistance film to form filaments of the metal. A forming operation is unnecessary in the metal filament-formed ReRAM because filaments are formed by causing the metal ions to diffuse and precipitate inside the insulating film, and the resistance value of the insulating film itself is not changed as in an oxygen deficiency-formed ReRAM. Operations are possible at relatively low currents; and because the number of filaments decreases as the memory cell is downscaled, the amount of current flowing in the entire memory cell can be reduced while ensuring the amount of current flowing in one filament. Thereby, the amount of current can be scaled as the memory cell is downscaled.
However, the metal filament-formed ReRAM has low current operations (high resistance operations); and the filaments themselves are fine and easily decompose. Therefore, the fluctuation of the switching operation is large; and the state retention characteristics degrade. Compared to a ReRAM using a transition metal oxide, the operating current is small; but the current density undesirably increases and electromigration is undesirably promoted as the interconnects are made finer by downscaling. Electromigration is one type of diffusion phenomenon due to the interaction between the metal atoms inside the metal interconnect and the electrons flowing through the metal interconnect. When the current is caused to flow in the interconnect, the metal atoms included in the interconnect are bombarded by the electrons, receive momentum in the direction in which the electron current flows, and move via vacancies. New vacancies occur in the spaces from which the metal atoms move, the vacancies grow into voids as the number of vacancies increases, and the effective cross-sectional area of the metal interconnect decreases. As a result, the current density locally increases; the temperature increases due to Joule heat; the occurrence of the vacancies is accelerated; and eventually the contact is broken.